With the advent of more and more plastic and electronic components on motorized vehicles, particularly automobiles, it is becoming more important to insulate these items from the hot components of the exhaust system. Presently, individual components or specific areas of the car are protected by heat shields or insulation, or are located a sufficient distance from the exhaust system to avoid heat. Heat shields or insulation can be costly when the item to be protected is large, such as a plastic gasoline tank, and it is not always feasible or practical to locate such items away from the exhaust system. A more economical approach is to insulate the source of the heat. In this case: the exhaust system component through which flows exhaust gas and which is heated by the exhaust gas. Exhaust system components include for example exhaust pipes as well as catalytic converters.
Pollution control devices may contain one or more catalysts, which are typically coated onto a substrate in the form of a monolithic structure. The monolithic structures are usually ceramic, although metal monoliths have been used. The catalyst(s) oxidize carbon monoxide and hydrocarbons; reduce the oxides of nitrogen, or a combination thereof in exhaust gases. Diesel particulate filters or traps are typically in the form of wall flow filters having a honeycombed monolithic structure made from porous crystalline ceramic materials. Alternate cells of the honeycombed monolithic structure are plugged so that the exhaust gas enters one cell, flows through the porous wall into another cell, and then exits the structure.
In common constructions of these pollution control devices, the monolithic structure is enclosed within a metal housing. Because the monolithic structure typically has a larger diameter than the exhaust pipe from the vehicle, the metal housing typically includes a transition zone between the inlet and/or outlet of the converter and the monolith. This transition zone, referred to as the end cone region, narrows from a diameter suitable for the monolithic structure to a diameter suitable for connection to an exhaust pipe. The end cone is usually conical in shape and can be provided on both the inlet and outlet side of the pollution control device.
The pollution control device typically must reach a certain temperature, e.g. 250° C. or above before they “light off” or begin to oxidize carbon monoxide and hydrocarbons. They are therefore preferably located close to the engine. Additionally, insulation is typically provided between the pollution control device and the housing of the converter and it is generally also preferred to insulate exhaust system components between the engine and the pollution control device so as to minimizes heat loss and therefore decreases the time for “light off” to occur. This is very important when the car is first started, especially in cold weather, to satisfy the increasingly stringent air quality standards.
Insulation is thus typically placed in the end cone region of the catalytic converter. The end cone region typically has a double-wall construction that includes an outer metal cone and an inner metal cone with a gap defined between the two cones. Insulation material can be placed in the gap between the inner and outer metal housings. The insulation can be in the form of a mat or as a three-dimensional form.
Many different insulation materials have been disclosed for use in exhaust system components. For example, U.S. Pat. No. 5,024,289 discloses an insulation material for use in the gap between the inner and outer wall of a double wall exhaust pipe. For use in insulation of end cone regions of a catalytic converter intumescent materials have been proposed that are typically also used to mount the pollution control device in the metal housing of the catalytic converter. These intumescent mat materials have been used at a lower assembled density than when used in mounting the pollution control device, since they need only function as an insulation and not as a holding mat. Such intumescent sheet materials are described in, for example, U.S. Pat. Nos. 3,916,057 and 4,305,992. These insulation materials usually contain vermiculite as the intumescent material and an organic binder to hold the mat together. For example, WO 98/50688 discloses the use of such intumescent material as insulation material in an end cone region of a catalytic converter. However, in application, the large vermiculite particles can vibrate relative to the mat forming holes in the mat, reducing its insulation property and in a worst case eventually leading to the destruction of the mat. Particles or pieces of mat escaping from the cone area can lead to plugging of the monolith cells, which in turn can lead to extremely high backpressure and failure of the catalytic converter.
Amorphous, refractory, ceramic fiber mats were also tried as insulation material for end cone regions of catalytic converters, but these mats generally lacked sufficient resiliency to provide long term durability as an end cone insulation. In addition, they also suffer from destruction of the mat upon vibrations typically encountered in motor vehicles. U.S. Pat. No. 5,250,269 describes a method to significantly increase the resiliency of refractory ceramic fibers by annealing them at high temperature. But even then the vibration resistance is still not satisfactory. Moreover, the annealing step adds further manufacturing steps making manufacturing less convenient and adding costs.
Polycrystalline fibers have also been used for end-cone insulation. These fibers generally perform well as insulative material in exhaust system components but they are much more expensive than the materials referred to above making their use in exhaust system components generally unattractive.
It would now be desired to find alternative insulation material that can be used to insulate various components of an exhaust system including for example an exhaust pipe or end cone of a catalytic converter. In particular it would be desirable to find insulation material that can be used in a gap between the walls of a double wall of the exhaust system component whereby the insulation material is less prone to destruction through vibrations that may occur when the exhaust system is in use in the motor vehicle. It will generally also be preferred that such insulation material is more environmentally friendly and preferably the insulation material can be produced in a convenient way and at a low cost. Finally, it would also be desirable that the insulation material can be used both in high temperature conditions such as for example encountered with common gasoline engines as well as at lower temperature conditions as may for example be found with diesel engines and in particular turbo diesel engines.